I've used a 6-ft card table with blow-molded plastic top as a ramp before. I'm going to look into using a folding version of a card table or make one from wood. There are folding "pet ramps" that could do the job, but they're overpriced for what you get.

The larger diameter wheels used by Lobster and SP will be helpful rolling onto the beginning of any ramp that doesn't have a "taper" at the ground.